Taking a cast of the jaw helps to determine the position of the lower jaw in relation to the upper jaw .

The lower jaw does not have a set position in relation to the upper jaw, the optimum position is continually 'recalculated' by the brain based on data collected by the many tiny receptors in the periodontal apparatus, the muscles and the maxillary joint. This way the chewing system can adapt very quickly to the wearing down and loss of teeth. You usually notice this phenomenon of variable lower jaw position in the morning, when the teeth do not fit as well against each other as they did the day before. After a few chewing motions, the brain, assisted by the sensitive receptors, is able to find the correct jaw position – and the day can begin.

If the lower jaw is „forced“ into an awkward position for example due to a faulty denture, this can cause pain in the face, the jaw and even headaches. If the mouth is fully equipped with teeth, the position of the jaw results from the fit of the teeth in relation to each other, if a patient has no teeth, or very few teeth, the technician needs information on how the upper and lower jaw are positioned in relation to each other in order to prepare a denture. A cast of the jaw provides the technician with the relevant information.

Here you can see a cast being made of a patient's jaw – the process is called the Dawson technique. This is all done with the patient lying down, the patient bites down on a cotton swab for 10 minutes in order for the computer – our brain – to get deprogrammed. After that the orthodontist takes the lower jaw of the patient using a special holder and closes the lower jaw – the orthodontist pushes the jaw up and to the front, so that the joint head comes to rest up towards the front. At the same time, the mouth of the patient contains a rubber gauge – a kind of squeezer, in which the ridges of the jaws can leave an impression. Finally, alginate casts are taken of the jaw and both squeezer and casts are sent to the technician.
The technician pours plaster into the casts and roughly positions the jaws using the squeezer – and now the technician can prepare a template of the jaw. With the aid of this template yet another cast can be made, instead of the squeezer the patient now wears the jaw template and the positioning of the jaws can be determined even more precisely. The process is the one used with the squeezer.

It is now assumed that the ideal position of the joint head in the joint socket is in the „upper front“, which is why when reconstructing the jaw an attempt is made to find this „ideal position“ of the lower jaw. A jaw cast need not be taken from patients who have an adequate amount of teeth and no serious complaints, since the way the teeth fit against each other provides the individual position of the jaw. This can diverge from the „ideal“, but this does not matter. The technique described here is one of many ways to determine the position of the lower jaw in relation to the upper jaw and to provide the technician with this information.

Earlier on, and even nowadays many patients' lower jaws are forced into the position they should have „according to the book“ using a denture or by grinding the teeth, which is absolutely pointless. A cast of the jaw is only necessary if there are not enough points of reference, for example if there are too few teeth or no teeth. Very rarely, but sometimes, a new position has to be determined for a patient who still has all their teeth – for example patients who have problems with their face or jaw. The rest of the teeth need to be sanded down to correspond to the new conditions.